javascript - use object in useEffect 2nd param without having to stringify it to JSON

Question

In JS two objects are not equals.

const a = {}, b = {};
console.log(a === b);

So I can't use an object in useEffect (React hooks) as a second parameter since it will always be considered as false (so it will re-render):

function MyComponent() {
  // ...
  useEffect(() => {
    // do something
  }, [myObject]) // <- this is the object that can change.
}

Doing this (code above), results in running effect everytime the component re-render, because object is considered not equal each time.

I can "hack" this by passing the object as a JSON stringified value, but it's a bit dirty IMO:

function MyComponent() {
  // ...
  useEffect(() => {
    // do something
  }, [JSON.stringify(myObject)]) // <- yuck

Is there a better way to do this and avoid unwanted calls of the effect?

Side note: the object has nested properties. The effects has to run on every change inside this object.

Answer 1

You could create a custom hook that keeps track of the previous dependency array in a ref and compares the objects with e.g. Lodash isEqual and only runs the provided function if they are not equal.

Example

const { useState, useEffect, useRef } = React;
const { isEqual } = _;

function useDeepEffect(fn, deps) {
  const isFirst = useRef(true);
  const prevDeps = useRef(deps);

  useEffect(() => {
    const isFirstEffect = isFirst.current;
    const isSame = prevDeps.current.every((obj, index) =>
      isEqual(obj, deps[index])
    );

    isFirst.current = false;
    prevDeps.current = deps;

    if (isFirstEffect || !isSame) {
      return fn();
    }
  }, deps);
}

function App() {
  const [state, setState] = useState({ foo: "foo" });

  useEffect(() => {
    setTimeout(() => setState({ foo: "foo" }), 1000);
    setTimeout(() => setState({ foo: "bar" }), 2000);
  }, []);

  useDeepEffect(() => {
    console.log("State changed!");
  }, [state]);

  return <div>{JSON.stringify(state)}</div>;
}

ReactDOM.render(<App />, document.getElementById("root"));

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.11/lodash.min.js"></script>
<script src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>

<div id="root"></div>